@andrewfeeney Gmail (like many other e-mail providers) also supports plus-addressing (account+foo@gmail.com) and the alternative domain googlemail.com (at least for users from Germany that registered before the dispute was resolved). So there are many ways to get an alias for an e-mail and I would not try to block that (i.e. Microsoft Exchange allowed plus-characters for the regular account names in the past).
Nevertheless, this sounds like something that could be included in a spam rating.